PWM current multiplication effect

[safe]

I call it a "death spiral" because the small motor is so grossly overloaded on a steep hill with a tall gear that whether the throttle is full or partial it heats up and starts to smoke. All that "throttle fiddling" only works when you have a lot of power to begin with. If your motor can't even pull the bike up the hill when at peak power (in the higher rpms usually about 3/4 of the top speed) then as you slow down under the load things just get worse and worse. If I had 3 times the motor to draw upon then I could with "brute force" pass out of the low rpms and get back up to where the motor is "happy" and running cool.

That's why I've said time and time again that the "big power" solution is equivalent to having gears because if you have three times as much "good power" (high efficiency power) then you can get out of the "death spiral".

Imagine a 140 lb bike, a 185 lb rider, and only a 750 Watt motor... without a low gear you won't get up hills very easily and any PWM "effect" gets lost in the motor being overwhelmed with excessive load... the PWM "effect" does not save the bike from the "death spiral"...

The lower rpms get, the faster the heating... so trying to keep the speed up so as to avoid dropping into the "death zone" part of the powerband is important..

 

[xyster]

That's an interesting point.

I'm not sure of the answer,

A philosophical aside: Isn't wonderful to learn that, having paid bouceau bucks and spent much time educating ourselves, we actually don't know very much at all (like how to spell "bookoo" ) and often can't definitively answer straight-forward questions pertaining to our areas of expertise?

Could this be what all the education was supposed to teach us?

 

[safe]

The torque needed to climb the hill, and therefore the motor current, will be nearly the same at 1/2 mph as it is at, say, 10mph (assuming the difference in wind and rolling resistance is negligible up to that speed).

But we've already seen the effects of low rpms if the current is "discontinuous". You get a heat effect that is related to the square of the current times the duty cycle. This has been the case with every ride I do, in that I can ride flat out in the powerband with full effiiciency all day long and not heat the motor, but get me onto one long hill that my powerband can't "peak" in (I can't take the load in that gear) and it drops down into the area where the current eventually goes "discontinuous" and the heat spikes to three or four times normal.

The bike becomes a "space heater" and that reduces efficiency which slows it even more... that's the "death spiral" that takes place...

The smaller motors are much more vulnerable to poor gearing than the "big iron"...

 

[safe]

This is basically what I "experience" on my rides if I use the "tall gearing" on hill climbs.

When I first hit the hill the rpms drop away, but the torque rises due to the PWM "effect" at low rpms. For a while you are thinking "oh great I can make this hill" and all seems fine. But slowly the heat rises and that lowers the ability of the motor to efficiently operate so it begins to lose power. The more power it loses the lower the rpms get and this increases the heating because you get into the "discontinuous" areas that are big heat producers. Finally the bike will no longer go forward and you are (I presume) seconds from disaster as your motor starts to smoke. The "death spiral" is complete. Any attempts to back off the power just hastens the rate at which the "death spiral" gets into the low rpms and that produces even more heat. (so the goal is to carry as much momentum up the hill as possible and hope it gets you over it before the heat stops you)

Again... this is "actual experience" verses "theory"... this is what actually happens on my bike...

Note: Your "power to weight ratio" is a big factor in this. When the weight is very high compared to the power you need to "finesse" the gears to get around and do things like carry your momentum through turns so that you can use that energy elsewhere. The power is small compared to the overall weight. On ANY machine with little weight and proportionally more power this would not occur. So this would NEVER occur for a "big iron" hub motor and might not occur on a little scooter that is geared really low.

My future bikes will have a wider gearing range (current is 200% and the future ones will have 300%) and will weigh a lot less. This should make the "death spiral" disappear.

The greatest irony in all of this is that since my bike is uniquely "vulnerable" to the PWM "effect" that I actually have to deal with it as a "real life" issue. The folks that are talking about how great it is might not have the configuration to really explore what it feels like and how well or not well it works in a practical sense.

 

[Matt Gruber]

safe
r u using instruments to measure this?
not making it up i hope

 

[safe]

This is how my bike behaves. You could propose alternative theories as to why things behave as they do, but my explanation seems to follow the physics pretty well.

Bikes with high power, low gears, and proportionally low weight will NOT experience the "death spiral" because the load is never high enough to make it happen.

The PWM "effect" (if used for short "spurts" of low end torque) is fine, but if you stay in that "zone" for very long you will see the increase in heat as predicted by the formulas. This formula seems to be correct and is proven by experience:

Heat = Current Squared Times the Duty Cycle

 

[xyster]

The greatest irony in all of this is that since my bike is uniquely "vulnerable" to the PWM "effect" that I actually have to deal with it as a "real life" issue. The folks that are talking about how great it is might not have the configuration to really explore what it feels like and how well or not well it works in a practical sense.

<p align="justify"><img src="http://upload.wikimedia.org/wikipedia/en/thumb/0/05/Lucy_and_the_psychiatric_help_booth.jpg/250px-Lucy_and_the_psychiatric_help_booth.jpg" align="right">You didn't have the sense to build your bike to do what you're expecting it to do, Safe. So now you're transferring your frustration at your own engineering shortcomings to a process, PWM, that makes EV's enjoyable and practical for the rest of us who did.

Five cents, please!</p>

 

[safe]

Safe. So now you're transferring your frustration at your own engineering shortcomings to a process, PWM, that makes EV's enjoyable and practical for the rest who did.

The "death spiral" does not occur with my "low gears". And in the future I will have a wider gear range. The fact of the matter is that I knew this by calculation before I built the bike. I've even been to the bike shop inquiring about a cassette style hub so that I could get a 300% gear range. (the old freewheel type can only cover 200%) But that means a new derailler, new chain, new "everything" and I've now mostly stopped development on this first prototype model.

So I've known all along of this problem.

The 8-speed will cure it because it has the 300% range...

 

[safe]

Throttle Usage and the PWM "effect"?

If there is "no load" to speak of on the motor then full throttle produces an accelleration that forces the controller to limit the current until the motor "releases" it's load by spinning it's way out of it. With "no load" a partial throttle setting will not incur the current limiting from the controller... so this means there's a difference between "full" and "partial" throttle.

However, if there is already a "load" on the motor because of something like a hill then the full throttle setting will force the controller to limit the current to it's current limit. With this same "load" a partial throttle will give the same result because the controller simply will see that the current is ALREADY MAXED OUT at low rpms so it again only allows as much current as is within the current limit.

So one has to be careful in thinking about what partial throttle can do for you. In the "fully loaded" condition the throttle becomes a virtual "on/off" switch.

In the end the PWM "effect" is mostly an rpms based thing when "fully loaded". A "fully loaded" motor at low rpms will be forced to current limit, which forces the duty cycle down, which makes the current "discontinuous" which induces the PWM "effect".

 

[Reid Welch]

I call it a "death spiral" because the small motor is so grossly overloaded on a steep hill with a tall gear that whether the throttle is full or partial it heats up and starts to smoke. All that "throttle fiddling" only works when you have a lot of power to begin with.....

Point: at such low motor speeds a motor generates almost no back-EMF. Without back EMF, the only limit on the current draw is the windings' resistance and the throttle setting.

So, the nearly-stalled motor is a resistive load.
The happy-speed motor punches back at the voltage, limiting current unless the throttle overules the motor's natural desire to self-limit.

Motors seem to want to live. They're almost like people. Push, and they push back.

humor time; how about a slogan all you ebreakers?


End Motor Cruelty. Throttle Yourself

 

[xyster]

End Motor Cruelty. Throttle Yourself

As I recall, that was Surgeon General Jocelyn Elder's motto during the Clinton years.

 

[TylerDurden]

Slogan:

If you think yer current is warped, you should meet Reid, the Ambassador of the Pushy Motor-People.

 

[safe]

End Motor Cruelty. Throttle Yourself

But if the hill is steep enough the load is too high and no throttle position can remedy the situation. That was the point... that throttle control cannot prevent the "death spiral".

The ways to prevent the "death spiral" are:

1. Reduce the load by reducing the weight if you are on a hill.

2. Change the gearing so that the motor doesn't have to do all the work at once. (it spreads the load out over time)

3. Use more power so that you can literally "lift" the bike out of the load condition and into the smooth running range.

Otherwise, if you REALLY are at the load limit then the controller will cut back on the duty cycle (that's it's job) which will create the PWM "effect" which increases the current which would normally would mean that heat increases as the square of current, but (since we are so well educated on the subject) in fact the squared value for current also gets multiplied by the duty cycle. But in the end you still get more heat.... just not as much as a "worst case scenario" would suppose.

Throttle control CANNOT reverse the "death spiral". (on a steep hill)

(and you live in Florida.. what the heck do you know about hills? )

 

[fechter]

For a given gearing, weight and power level, there's a limit to how steep a hill you can go up.

Know your limits.

For idiots and consumer goods, the current limit could be set at a level low enough to pretty much guarantee that it will never burn up. Of course, it will be gutless too.

A high temp foldback on the current limiter would be the best.

 

[Matt Gruber]

safe
r u using instruments to measure this?
not making it up i hope

1. i am amazed how powerful my 500w MY1020 is.(mbike is 300# loaded)
2. safe smoked his 750 and some windngs are likely shorted
3. no DVM, no temp sensor, = new motor needed
4. safe can't try part throttle because it is too hard to hold in the right spot(i sure couldn't). 1st time i hit the hill i came home and built my pot limiter. L thumb control.
5. safe has lots of questions and that is a good thing. But requires a fully instrumented motor dyno or years of experience to get the facts.

 

[safe]

2. safe smoked his 750 and some windngs are likely shorted
3. no DVM, no temp sensor, = new motor needed
4. safe can't try part throttle because it is too hard to hold in the right spot(i sure couldn't).

The motor appears to be okay, once it cooled down it went back to normal. All the old abilities are still there. But I did learn the lesson that heat is to be avoided. (it was a real scare that's for sure)

Part Throttle CANNOT "save the day" if a bike is geared too high. Think of it this way... if the bike could make it up a hill with 100 lbs then great. Now add on 200 lbs... maybe it still makes it. Now 300 lbs. 400 lbs. 500 lbs. At some point you reach a power-to-weight ratio where the motor cannot sustain the load indefinitely. It's at this point that the "death spiral" takes place because under the heavy load the heat build up increases. At some point the rpms drop into the really low rpms and the "discontinuous" area of PWM starts to change the heating to an even more rapid rate. So the "death spiral" has a sort of exponential end to it where things simply get worse and worse.

The only remedies are as stated:

1. Less weight.
2. Lower gearing.
3. More power.

Philosophically you have to get past the "in my experience" case and think about how "others" (like myself) have their machines set up. I have a lot of weight (86 lbs of batteries) and a very small motor and manage a lot of performance because of gears. However, those same gears that can give me a truly "blessed ride" of pleasure can also fail me at the point of greatest need if they lack the range of gearing needed.

The moment I hit a hill of a certain slope the "clock" starts ticking as the heat increases. If I can't crest the hill before the worst of the PWM "discontinuous" current "effect" kicks in and the heat goes off the charts then I'm going to have to push the bike up the rest of the hill. Fortunately I carry a shop cloth so that I can always stop on the side of the road and physically switch from my 22 tooth (front) sprocket down to the 14 tooth one. In the 14 tooth sprocket I can climb just about anything, there are no hills in my town that I can't climb with the 14 tooth sprocket. The problem though is that the 14 tooth only gives me the hillclimb gearing like the chart below... 31.4 actual / 35.7 max... verses the other chart for top speed... 44.3 actual / 56.0 max...

The "death spiral" is real and due to heating and there is a chain reaction quality to how it takes place. There is no partial throttle solution because that brings the rpms lower which rapidly increases the heat. Remember, for the "discontinuous" area of PWM the heat grows at a rate of:

Heat = Current Squared Times the Duty Cycle.

In order to recreate the experiment on your bike I would recommend ADDING 100 lbs of dead weight (do you have some lifting weights around the house?) and then going to a hill that is very steep and see if that is enough weight to trigger it. If you have gears all you have to do is gear the bike taller. If you have a hub motor you might never have known how the limits of power really work because they design the "gearing" (so to speak) on a hub motor to be really low.

Try the 100 lbs test and then get back to me... (don't forget your goal is to "fail the hill"... if you make it to the top you don't have enough weight yet)

 

[Matt Gruber]

That's an interesting point.

I'm not sure of the answer, but my intuition tells me that if I did two runs on the same hill, one at full throttle, and one going very slow, like 5mph, that the motor would be hotter after the fast run.

For sure, the slow run would be drawing less current from the batteries, so Peukert losses or battery resistance losses, would be much less, which would consume less available energy from the battery.

On the slow run, the motor would have more time to dissipate the wasted heat, so it could dissipate more energy without getting too hot. I'm not sure if that compensates for the additional inefficiency.

If you take the example to the extreme, imagine going up a hill at like 1/2 mph. The torque needed to climb the hill, and therefore the motor current, will be nearly the same at 1/2 mph as it is at, say, 10mph (assuming the difference in wind and rolling resistance is negligible up to that speed).

To complete climbing the hill, the same total energy is needed in both cases. The amps going through the motor is the same in both cases. It takes 20 times longer in the slow case, so it would seem like it should create 20 times more wasted energy.

I'm not sure how you would test this. A thermometer in the windings might do it.

I got the 60% from the a site discussing golf carts.
There was nothing there about other speeds like crawling 5 mph or less.
My goal is to stay UNDER or near the continuous power rating when possible.
I've read (Honk maybe) that once motor rpm falls below 50% on a too steep hill, it is dead meat.

Armed with this, and other info, i have a procedure to deal with steep hills. That includes(not used yet) turning around if it is too long, or resting part way to cool off.

 

[fechter]

I have a fair amount of experience stalling motors on hills. If the current limiter is properly set and working, when you attempt to exceed the hill climbing capability, the vehicle slows down until it eventually stalls. It shouldn't take a rocket scientist to figure out when a stall occurs, and you let off the throttle. All this should happen before smoke starts coming out.

It's more fun to have the current limiter set at a level that's higher than the "idiot proof" setting. When the motor's cold, you can pound it with 2 -3 times more current than it can take continuously. As long as you monitor the temperature and know when to back off, this is perfectly fine for the motor.

One problem is motor temperature is affected by a bunch of other things like ambient temperature, cooling, and thermal inertia of the motor parts. If you've been riding at moderate power levels for a long period of time, the heat has fully spread throught the motor parts and everything will be pretty hot (heat soak). If you hit a big hill, it gets even hotter. If you did the same hill with a cold motor, it would have much more time before it got too hot.

Motor temperature feedback to the current limiter circuit would be the ideal solution. This would be fairly easy with a brushless motor, since you could put a thermistor right in the windings. With a brushed motor, it's a bit less optimal since you can't monitor the winding temperature direclty (unless you use an infrared sensor). Any other temp sensor would lag the armature temperature long enough to need some kind of compensation.

This way you could pound the motor with high current until it gets warm, then the current limit would start dropping as you approached the maximum allowable temperature. Both fun and idiot resistant.

 

[Matt Gruber]

fechter
clearly, 20 yrs from now ebikes will have motor temp compensation.
hopefully SOONER!

 

[Matt Gruber]

safe
i'd like to evaluate your bike, but you are just too stubborn to take seriously.

You remind me of Don C.

In H.S. Don C. was sure in NJ if he drove his car every day it wouldn't need antifreeze. He was 17 and able to adapt quickly. Still he had 2 blocks crack before he changed his mind.

 

[safe]

This way you could pound the motor with high current until it gets warm, then the current limit would start dropping as you approached the maximum allowable temperature. Both fun and idiot resistant.

I was just thinking about the "Current Based Throttle" idea and how if you were to be using something like that you really can protect against overheating because the partial throttle doesn't just lower the voltage, but also the current limit. That way you know that if you are running at full throttle that means the potential for "heat", but at partial throttle you know (for sure) that there will be less of it.

With the "perfect" low current limit controller you have the best foundation to begin with, but "accumulated heat" over the course of a ride does add up. Having a "force fed" air supply into your motor all the time would eliminate the heat buildup and mean with every new hill you are always starting with a "fresh motor".

On my motor if it does get hot it takes about half an hour to cool off...

 

[safe]

...but you are just too stubborn to take seriously.

I'm a stubburn old man who will "stick to his guns" if he thinks he has something worth defending. My first project has been successful in some ways and less so in others. (inadequate gearing range has been the biggest problem since the beginning on this one) With each design (and more knowledge of the "tricks of the trade") the projects will get better and better with each incarnation.

Retired folks need a hobby... :wink:

 

[fechter]

Retired folks need a hobby... :wink:
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Damn, wish I could retire..

 

[safe]

Damn, wish I could retire..

If you had invested in Real Estate in the San Francisco area about 10 years ago and rode it up for about 6 -7 of those years and then sold you could retire like I did. I actually made more money in Real Estate than all the computer jobs put together. (well, about the same I guess) Take a 3 to 1 price differential of San Francisco to Missouri house prices and you don't need to work anymore. Plus I'll be getting an inheritance eventually too, so that makes it easier.

But I do miss all the fun stuff you can do in California... when you aren't stressed out from overwork that is... :wink:

Another factor is that in California the women are beautiful, lonely, and desperate so a relationship "pig" like me could have all the women I wanted. Here in the midwest the women are fat, repulsive, and spoiled and so the relationship picture here is terrible. (from my perspective it's like "role reversal", now the women get to be the "pig" and I have to be the "desperate" one) Supposedly the WORST place in the US to find women is Dallas, TX, but Missouri is very close at I think 4th worst. The midwest is no place to find a girlfriend that's for sure...

(if you are married already, then all the titillation of being in California is just frustrating)

 

[safe]

The Second Powerband

With all this talk of the PWM "effect" I really had to dedicate some riding to be sure of what I've been saying. While it's true that on a severely steep hill with too tall of a gear ratio the "death spiral" is the only result, I tried instead to see what happens on MODERATE hills.

If the hill is roughly of a steepness that in my first gear I would normally be able to "just about" pull that gear at full power, not far from full efficiency, but I'm just "slightly" loading the motor down then if you back off and let the rpms drop all the way down you CAN catch a very slow speed "second powerband" that doesn't heat the motor up that much.

So in a sense I'm being more "open minded" about the PWM "effect" in that I can see for people without gears how it is almost like having an extra "low gear". (it's waaaaay down there and not all that practical for "speed oriented" riding)

Normally I try to keep the bike in the "peak power" and "peak efficiency" areas because... well... I'm always searching for more speed. This "second powerband" is way, way, way, down low and yet it does make me think that in certain circumstances it adds benefit to some people.

The "death spiral" is still there for the "killer hills" on my bike, but I'm more "clear" now on exactly what you "hub people" get all excited about with PWM.

So I think I "see" the other sides perspective better now...

(and on a "killer hill" if I have to get out and walk alongside the bike I won't worry so much about using a little teeny, tiny, bit of throttle just to make the bike move)